Field of the Invention
The invention relates to a continuous synthesis process for the preparation of high quality indium phosphide/zinc sulfide core/shell semiconduting nanocrystals in particular quantum dots (QD).
Description of Related Art
Colloidal semiconductor nanocrystals have attracted intense interest during the past two decades, owing to their unique chemical, physical, electronic properties, which brings many potential technological applications in biological labeling, LEDs, lasers, photovoltaics and sensors, etc. Among all II-VI and III-V semiconductors, InP is probably the only system which offers a compatible, or even broader emission color range than the CdSe-based system but eliminating intrinsic toxicity since InP contains neither Class A elements (Cd, Hg, Pb), nor class B elements (e.g. As, Se) (Xie et al. J. AM. CHEM. SOC., 2007, 129, 15432; Reiss et al. J. AM. CHEM. SOC., 2008, 130, 11588). Nevertheless, synthesis of high-quality InP remains challenging. The existing problems include among others low photoluminescence quantum yield, poor size distribution, sensitive precursors and poor control of the stability. The synthesis procedure of InP is also more delicate compared to the one of CdSe-based QDs partially due the highly sensitive phosphine precursors (Nann et al. J. AM. CHEM. SOC., 2006, 128, 1054; Nann et al. J. Mater. Chem., 2008, 18, 2653).
Making use of the micro-reaction technique based continuous synthesis facility; we are exploring solutions for these problems. In recent years, micro-reaction technology has emerged as an alternative for the synthesis of high-quality nanoparticles due to the advantages that this technology provides: Precise control of the reaction parameters like temperature profiles, miniaturized reaction volume, fast reaction speed and its parallel operation possibility may lead to a scalable process of production of various nanoparticles (Blackmond et al. Angew. Chem. Int. Ed. 2010, 49, 2478; WO2008061632, WO2002053810). Besides, the enhanced heat transfer and mixing efficiencies in the micro-channel allow elevating the precursor reactant concentration above the nucleation threshold in a very short period of time, and the burst of nucleation can be promoted by raised temperature providing a reliable strategy to separate the nucleation and growth phases during the heating stage required to achieve a better particle size distribution.
Although WO2008061632 and WO2002053810 mention the continuous preparation of binary semi-conducting nanoparticles using micro-reaction technique should be applicable for the preparation of InP nanoparticles, the methods were only exemplified with Cd-based cores and needed to be modified to solve the particular problems of the preparation of nanoparticles comprising InP core.
There was a need for a method for the preparation of InP nanocrystals leading to high photoluminescence quantum yield, narrow size distribution and enhanced stability despite the use of sensitive precursors.
The stabilization of InP nanoparticle using a protective ZnS shell is known to enhance environmental stability, chemical and photochemical stability, reduced self-quenching characteristics, and the like. In particular Peng et al. [J. Am. Chem. Soc., 2007, 12, 15432-15433] describe the “one-pot” preparation of InP/ZnS core shell nanoparticles wherein the size range reachable for one reaction was readily tuned by the concentration of amines, the concentration and chain length of fatty acids in particular myristic acid used for dissolving In(Ac)3 precursor, and the reaction temperatures (below 200° C.). The most convenient method in tuning the size was by varying the concentration of the fatty acid.
The fluorescence properties of semiconduting nanocrystals are known to depend on the quantity and quality of surface defects. Surface capping ligands are known to be crucial to achieve high quantum yield nanocrystals. A metal rich surface is also advantageous for a good saturation of surface defects. The typical capping ligands for II-VI semiconductor nanocrystals, such as trioctyl phosphine oxide (TOPO) and trioctyl phosphine (TOP) have stronger coordinating strength towards indium than for example, cadmium. It was found that InP nanoclusters dissolve rapidly in the presence of these ligands at temperature above 200° C., which leads to unstable initial nuclei, more intrinsic defects and slower crystallization process. Nann et al. teach that only weak or non-coordinating ligands added to the core reaction mixture are able to prevent the negative influence of strong coordinating ligands and an excess of indium precursor is necessary to avoid nanocrystal aggregation due to the lack of surface ligands. The excess of indium precursor was shown not only to support rapid nucleation, but also to provide an indium rich surface with reduced surface defects [Nann et al. J. Mater. Chem., 2008, 18, 2653]. Nann et al. also teach that addition of stable zinc carboxylate into the reaction mixture does not result in lattice doping due to intrinsic low reactivity of zinc carboxylate compared with the other reagents involved in the synthesis but that zinc helps passivating the InP surface by coupling to the dangling phosphine bonds, hence enhancing the photoluminescence quantum yield of InP nanocrystals significantly. Furthermore, the addition of zinc carboxylates was shown to stabilize the QDs' surfaces and reduce the critical nuclei size as shown by shift of the photoluminescence emission wavelength to the blue observed with increasing concentration of initial zinc carboxylate [Nann et al. J. Mater. Chem., 2008, 18, 2653]. Typically tris(trimethylsilyl) phosphine (TTSP) is used as P-precursor for the preparation of InP core [Nann et al. J. Mater. Chem., 2008, 18, 2653]. However TTSP is sensitive to oxidation and requires intensive degasing under inert atmosphere before use and throughout reaction process, so handling during batch production is delicate, time consuming and costs are high.
There was a need for a method for the preparation of InP/ZnS core/shell nanocrystals leading to high photoluminescence quantum yield, narrow size distribution and enhanced stability despite the use of sensitive precursors, wherein production costs are reduced.